Emissions control of sulfur dioxide and nitric oxide in flue gas streams resulting from the combustion of carbon-based fuels in power plants is an advancing art. There is a burgeoning need for an efficient and inexpensive process for abating such pollutant gases to meet ever increasing environmental concerns and the growing need to utilize plentiful, but pollutant-producing fuels, such as coal, to replace diminishing petroleum reserves.
In a widely employed practice, sulfur dioxide-containing flue gases are scrubbed with an aqueous lime or limestone slurry. Sulfur dioxide is removed by precipitation of hydrated CaSO.sub.4 and CaSO.sub.3 salts. A common variation of this process utilizes SO.sub.2 sorbents such as sodium carbonates to produce a bisulfite intermediate. The sorbent may be subsequently regenerated by contacting the bisulfite with lime or limestone. The insoluble calcium salts produced thereby are typically disposed of in landfills.
Metal chelates, and ferrous chelates in particular, have been introduced to address the problem of NO abatement such as, for example, in U. S. Pat. Nos. 4,732,744 to Chang et al.; 4,612,175 to Harkness et al.; 4,708,854 to Grinstead; 4,615,780 to Walker; and 4,126,529 to DeBerry. In these references, SO.sub.2 is scrubbed concurrently by a SO.sub.2 sorbent such as water soluble sulfite, bisulfite and carbonate salts. The SO.sub.2 is typically removed as a sulfite or sulfate waste stream such as, for example, in Chang by a crystallization separation process, in Walker and DeBerry by electrodialysis or in Grinstead by ultrafiltration or dialysis.
U.S. Pat. No. 4,347,227 to Hass et al. discloses an aqueous solution of formic or oxalic acid and an alkanolamine containing a tertiary amine group used to scrub SO.sub.2 from the flue gas. The SO.sub.2 may be recovered in an absorbent regeneration stage, but this process requires a 2-stage absorption step.
U. S. Pat. Nos. 4,783,327 to Treybig et al. and 4,530,708 to Jones et al. both disclose processes for selectively removing and recovering sulfur dioxide from a gas stream by absorbing the SO.sub.2 in an aqueous solution containing piperazinones. However, neither process discloses chelates for NO abatement.
In commonly assigned application Ser. No. 569,117, filed Aug. 16, 1990, entitled "Process for Absorption of Sulfur Compounds from Fluids," (applicants Dane Chang and Steven H. Christiansen), there is disclosed a process wherein sulfur dioxide is removed from a fluid containing SO.sub.2 by employing as an absorbent for the SO.sub.2 an aqueous solution of a compound represented by the formula: ##STR1## wherein each Y is independently --(CR.sup.2.sub.2)--, --(CR.sup.2.sub.2)= or --(C.dbd.O)-- and each R.sup.1 and R.sup.2 is independently hydrogen, an alkyl group; a group; an aldehyde group; a carboxylic acid group or salt thereof; an alkyl group containing at least one carboxylic ester, a carboxylic acid or salt thereof, ether, aldehyde, ketone, or sulfoxide group; wherein the compound has at least one carbonyl group. Representative of such absorbents are 2-piperidone and ethyl piperidone 2-carboxylate.
In commonly assigned application Ser. No. 569,120, now U.S. Pat. No. 5,098,681, filed Aug. 16, 1990, entitled "Process for Absorption of Sulfur Compounds from Fluids" (applicants Steven H. Christiansen, Dane Chang and Druce K. Crump), there is described a process wherein sulfur dioxide is removed from a fluid containing SO.sub.2 by employing as an absorbent therefor an aqueous solution of a compound represented by the formulae: ##STR2## wherein each R.sup.1 or R.sup.2 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic acid or salt group; an alkyl group containing at least one carboxylic ester, carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide; and wherein at least R.sup.1 or R.sup.2 is carbonyl-containing group, such as an aldehyde group, a carboxylic acid containing group, a carboxyl ester group, or a ketone-containing group; or ##STR3## wherein each R.sup.3 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic ester, a carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide group. Representative of such absorbents are ethyl-1-piperazine carboxylate, 1,4-diformyl piperazine, 1-succinyl piperazine, glycine anhydride, and 1,4-dimethyl-2,5-piperazinedione.
In commonly assigned application Ser. No. 569,091, filed Aug. 16, 1990, entitled "Process for Absorption of Sulfur Compounds from Fluids" (applicants Steven H. Christiansen, Dane Chang and David A. Wilson), there is described a process wherein sulfur dioxide is removed from a fluid containing SO.sub.2 by employing as an absorbent therefor an aqueous solution of compound represented by the formula: ##STR4## wherein X is --O--, --NR.sup.1 --, or --N.dbd.; each Y is independently --(CR.sup.2.sub.2)--, --(C.dbd.O)--, --O--, NR.sup.1 --, --N.dbd., or --C(R.sup.2).dbd.; each R.sup.1 and R.sup.2 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic acid or salt group; or an alkyl group containing an aldehyde group, a carboxylic acid or salt group, ketone, carboxylic ester, ether, or sulfoxide group; and m is an integer preferably of from 2 to about 4. Representative of such absorbents are hydantoin, , N.sup.2,N.sup.3 -dimethylpropyleneurea, trialkyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, and 2-oxazolidone.
In commonly assigned application Ser. No. 569,118, filed Aug. 16, 1990, entitled "Process for Absorption of Sulfur Compounds from Fluids" (applicants Dane Chang, Steven H. Christiansen and David A. Wilson), there is described a process for removing SO.sub.2 from a fluid containing SO.sub.2 by employing as an absorbent for the SO.sub.2 an admixture of water and a compound represented by the formula: ##STR5## wherein each R.sup.1, R.sup.2 or R.sup.3 is independently hydrogen; an alkyl group; a carboxylic acid group; a hydroxyalkyl group; an aldehyde group; or an alkyl group containing a carboxylic ester, a carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide group; wherein at least one of R.sup.1 and R.sup.' is a carboxymethyl group and at least one of R.sup.1 and R.sup.' is hydrogen, preferably at least one R.sup.1 is carboxymethyl and at least one R.sup.' is hydrogen. Representative of such absorbents is symmetrical ethylenediaminediacetic acid.
U.S. Pat. No. 4,863,705 to Epperly et al. describes the use of a 5 or 6 member heterocyclic hydrocarbon having at least 1 cyclic nitrogen as a treating agent for reducing the concentration of nitrogen oxides in a flue gas.
It is desirable that a process be devised for the efficient, simultaneous removal of SO.sub.2 and NO from flue gas streams wherein SO.sub.2 may be recovered, the absorbent may be regenerated, and heat stable salts may be maintained at acceptable concentrations, with minimum waste formation.